Method and apparatus for applying a rectilinear biphasic power waveform to a load

ABSTRACT

A system and method to deliver a therapeutic quantity of energy to a patient. The system includes a capacitor having a rated energy storage capacity substantially equal to the therapeutic quantity of energy, a boost converter coupled with the capacitor and constructed to release energy from the capacitor at a substantially constant current for a time interval, and an H-bridge circuit coupled with the boost converter and constructed to apply the substantially constant current in a biphasic voltage waveform to the patient. The method includes storing a quantity of energy substantially equal to the therapeutic quantity of energy in a capacitor, releasing the quantity of energy at a relatively constant current during a time interval using a boost converter coupled with the capacitor, and delivering a portion of the quantity energy in a direction to the patient using an H-bridge circuit coupled with the boost converter.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation under 35 U.S.C. §120 of U.S. patentapplication Ser. No. 15/074,947, filed on Mar. 18, 2016, which is acontinuation of U.S. patent application Ser. No. 14/547,865, filed onNov. 19, 2014 and issued as U.S. Pat. No. 9,289,617, which is acontinuation of U.S. patent application Ser. No. 13/967,878, filed onAug. 15, 2013 and issued as U.S. Pat. No. 8,909,335, which claims thebenefit of priority under 35 U.S.C. §119(e) to U.S. ProvisionalApplication Ser. No. 61/691,137 filed on Aug. 20, 2012. All subjectmatter set forth in each of the above referenced applications is herebyincorporated by reference in their entirety into the present applicationas if fully set forth herein.

BACKGROUND Field

Aspects of embodiments relate generally to methods and apparatus forapplying a selected energy impulse to a load without exceeding a safepower level. More particularly, aspects of embodiments relate toapplying electrical energy impulses to a patient for therapeutic medicalpurposes. Even more particularly, aspects of embodiments relate to suchmethods and apparatus as used in heart defibrillators and/or pacingdevices.

Discussion of Related Art

Current defibrillator technology stores electrical energy on acapacitor, a passive energy storage element, preparatory to applying atimed, e.g., 10 msec, rectilinear, biphasic energy impulse of a desiredmagnitude to a patient. In a known defibrillator, energy is applied as acurrent from the capacitor through the patient in a first phase, i.e., afirst direction, for the first 6 msec of the energy impulse, and then asa current also from the capacitor, but through the patient in anopposite phase to the first phase, i.e., in a second direction oppositeto the first direction, for the remaining 4 msec of the energy impulse.In order to accommodate a wide range of patients and operatingconditions, especially the voltage droop that occurs as energy istransferred out of the capacitor, the capacitor is charged to a higherlevel of energy than required to produce the desired energy delivery.The above-described, known defibrillator incorporates a resistor networkinto which excess energy is dissipated by diverting a portion of thecurrent from being delivered to the patient when sensors detect thatpower levels may be dissipated in the patient that exceed safe powerlevels.

SUMMARY

According to aspects of an embodiment, a method of applying arectilinear biphasic electric power waveform to deliver a therapeuticquantity of energy to treat a patient presenting an electrical load isprovided. The method comprises storing a quantity of energysubstantially equal to and without substantially exceeding thetherapeutic quantity of energy on a capacitor, and releasing the storedenergy during a first interval in a first direction through the loadpresented by the patient, in a controlled manner using a boostconverter. The method may further comprise releasing the stored energyduring a second interval in a second direction through the loadpresented by the patient. The method may yet further comprisesubstantially exhausting the stored energy over the first interval andthe second interval combined without exceeding a predetermined maximumsafe power level when the load presented by the patient is betweenapproximately 25Ω and 200Ω. The method may even yet further comprisereleasing a portion of the stored energy from the capacitor into aninductor; releasing the portion of the stored energy from the inductorinto the load; and controlling the releasing of the portion of energyinto the inductor and into the load in an alternating sequence so as toproduce a substantially even flow of energy into the load. According toother aspects of the embodiment, the therapeutic quantity of energy andthe first interval are selected to pace a patient whose heart requirespacing impulses. According to yet other aspects of the embodiment, thetherapeutic quantity of energy and the first interval are selected todefibrillate a patient whose heart is in fibrillation.

According to aspects of another embodiment, a system for applying arectilinear biphasic electric power waveform to deliver a therapeuticquantity of energy to treat a patient presenting an electrical load isprovided. The system comprises a capacitor having a rated energy storagecapacity substantially equal to the therapeutic quantity of energy, aboost converter constructed and arranged to meter energy out of thecapacitor as a substantially constant current while a voltage across thecapacitor droops due to decreasing energy stored on the capacitor, andan H-bridge circuit constructed and arranged to apply the current to thepatient in the rectilinear biphasic electric power waveform. The systemmay further comprise a controller that controls for a 10 msec combinedfirst and second interval, and a 6 msec first interval.

According to aspects of an embodiment, a system to deliver a therapeuticquantity of energy to a patient load is provided. The system comprisescapacitor having a rated energy storage capacity substantially equal tothe therapeutic quantity of energy, a boost converter coupled with thecapacitor and constructed to release energy from the capacitor at asubstantially constant current for a time interval, and an H-bridgecircuit coupled with the boost converter and constructed to apply thesubstantially constant current in a biphasic voltage waveform to thepatient load.

According to an embodiment, the boost converter comprises an inductorcoupled with the capacitor, a current sensing network, and a solid-stateswitch coupled between the inductor and the current sensing network. Theboost converter may further comprise a controller circuit coupled withthe solid state switch and the current sensing network and constructedto cycle the solid state switch. According to other aspects of theembodiment, the current sensing network is constructed to receive acurrent profile and compare the current profile with a received currentfrom the solid state switch.

According to an embodiment, the H-bridge circuit comprises a pluralityof switches, each of the plurality of switches including a circuitconstructed to control the switch and to receive a phase profile havinga first phase and a second phase. According to other aspects of theembodiment, the H-bridge circuit may further comprise an invertercoupled with at least two of the plurality of switches to invert thephase profile. At least two switches of the plurality of switches may beconfigured to be in an open state during the first phase and in a closedstate during the second phase.

According to an embodiment, the boost converter circuit is furtherconstructed to compensate for voltage droop on the capacitor andvariation in the patient load over the time interval. According to anembodiment, the therapeutic quantity of energy and the time interval areselected to defibrillate a patient whose heart is in fibrillation.According to an embodiment, the therapeutic quantity of energy and thetime interval are selected to pace a patient whose heart requires pacingimpulses.

According to aspects of an embodiment, a method of delivering atherapeutic quantity of energy to a patient load is provided. The methodcomprises storing a quantity of energy substantially equal to thetherapeutic quantity of energy in a capacitor, releasing the quantity ofenergy at a relatively constant current during a time interval using aboost converter coupled with the capacitor, and delivering a firstportion of the quantity energy in a first direction to the patient loadusing an H-bridge circuit coupled with the boost converter. According toan embodiment, the method further comprises delivering a second portionof the quantity of energy in a second direction to the patient loadusing the H-bridge circuit.

According to an embodiment, releasing the quantity of the stored energyincludes transferring energy to an inductor coupled with the capacitor,and sensing the amount of current through a solid state switch coupledbetween the inductor and a current sensing network. According to anembodiment, releasing the quantity of energy includes cycling the solidstate switch using a controller circuit coupled with the solid stateswitch and the current sensing network. According to an embodiment,releasing the quantity of energy further includes receiving a currentprofile and comparing the current profile with the amount of currentthrough the solid state switch using the current sensing network.

According to an embodiment, the H-bridge circuit comprises a pluralityof switches and wherein delivering a first portion of the quantityenergy in a first direction and a second portion of the quantity ofenergy in a second direction includes controlling the plurality ofswitches. According to an embodiment, controlling the plurality ofswitches includes receiving a phase profile having a first phase and asecond phase. According to other aspects of an embodiment, controllingthe plurality of switches further includes changing a state of at least4 switches of the plurality of switches in response to receiving achange in the phase profile from the first phase to the second phase.

According to an embodiment, releasing the quantity of energy includescompensating for voltage droop on the capacitor and variation in patientload impedance over the time interval. According to an embodiment, themethod further comprises determining the therapeutic quantity of energyand the time interval to defibrillate a patient whose heart is infibrillation. According to an embodiment, the method further comprisesdetermining the therapeutic quantity of energy and the time interval topace a patient whose heart requires pacing impulses.

According to aspects of yet another embodiment, a method of maintaininga target power flow from a charge storage device to a patient load whilevoltage on the charge storage device droops, comprises inserting a boostconverter between the charge storage device and the patient load tomaintain power flow. The method may further comprise controlling acurrent delivered by the boost converter so as to compensate for voltagedroop on the charge storage device and so as to compensate for variationin patient load impedance over time.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1 is a schematic drawing of a circuit for delivering a rectilinearbiphasic electric power waveform to deliver a therapeutic quantity ofenergy to treat a patient presenting an electrical load;

FIG. 2 is a graph of electrical waveforms produced by the circuit ofFIG. 1 over a period of time with a patient load of 25Ω;

FIG. 3 is a graph of electrical waveforms produced by the circuit ofFIG. 1 over a period of time with a patient load of 50Ω;

FIG. 4 is a graph of electrical waveforms produced by the circuit ofFIG. 1 over a period of time with a patient load of 100Ω;

FIG. 5 is a graph of electrical waveforms produced by the circuit ofFIG. 1 over a period of time with a patient load of 150Ω;

FIG. 6 is a graph of electrical waveforms produced by the circuit ofFIG. 1 over a period of time with a patient load of 200Ω;

FIGS. 7A-F illustrate various current profiles and phase profiles thatmay be used with the circuit of FIG. 1 to deliver a variety of differentdefibrillating waveforms to the body of a patient;

FIG. 8 is a schematic drawing of a drive circuit for switches used in anH-bridge sub-circuit of the circuit of FIG. 1;

FIG. 9 is a graph of electrical waveforms produced by the circuit ofFIG. 1 with a patient load of 25Ω, when programmed for pacer mode; and

FIG. 10 is a graph of electrical waveforms produced by the circuit ofFIG. 1 with a patient load of 300Ω, when programmed for pacer mode.

DETAILED DESCRIPTION

This invention is not limited in its application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of other embodiments and of being practiced or of being carriedout in various ways. Also, the phraseology and terminology used hereinis for the purpose of description and should not be regarded aslimiting. The use of “including,” “comprising,” “having,” “containing,”“involving,” and variations thereof herein is meant to encompass theitems listed thereafter and equivalents thereof as well as additionalitems.

As noted in the BACKGROUND section, defibrillators are devices thatdeliver a desired quantity of energy to a patient without exceeding asafe power level. Energy is simply power delivered to a load over aperiod of time:

Pt = E; or ${P = \frac{E}{t}};$

where P represents power in Watts, E represents energy in Joules, and trepresents the period of time in seconds over which the energy isdelivered. When electrical energy is dissipated in a simple resistiveload, that is, one which resists a flow of electrical current when avoltage is applied, power may be expressed in terms of the voltageapplied to the load, voltage being a measure of electrical pressureacross the load, and current through the load, current being a measureof movement of charge through the load. Electrical power is:

P = VI; or P = I²R; or ${P = \frac{V^{2}}{R}};$

where V represents voltage in Volts, I represents current in Amperes,and R represents the resistance of the load in Ohms.

Defibrillators store the desired quantity of electrical energy on acapacitor, as a charge. Storing a charge on a capacitor causes a voltageto appear across the terminals of the capacitor. When a user of adefibrillator applies a therapeutic shock to a patient, the electricalenergy stored on the capacitor is released through the patient, whosebody provides substantially a simple resistive load in which the energyis dissipated. As the capacitor supplies energy to the load, the chargeon the capacitor decreases, and so the voltage appearing across thecapacitor also decreases. As voltage decreases, or sags, the currentdriven through the load also decreases. Applying any of the definitionsof electrical power given above, it is observed that the power, P₀,delivered by the capacitor at the beginning of a therapeutic shock of adefined magnitude, E₀, is greater than the power, P_(N), delivered bythe capacitor at the end of the therapeutic shock because the voltage onthe capacitor sags as the charge on the capacitor is depleted bysupplying current to the patient.

Conventionally, in order to accommodate the voltage sag, whiledelivering a constant, desired maximum power level until the desiredenergy impulse has been delivered, the size of the capacitor is selectedto provide the desired energy impulse to a worst-case load at the end ofthe energy impulse. For these purposes, a worst-case load may beconsidered to be one at a lower end of an expected resistance range,since such a load will require a larger current to maintain a constantpower level during the energy impulse. Such a design requires acapacitor that, when charged to a level that yields the desired energyimpulse, dissipates in the patient a power level in excess of thatdesired during the initial portion of the energy impulse. As previouslyexplained, during times of excess power delivery, the excess energy issimply dissipated into resistors so as to reduce to desired maximumlevels the power delivered to the load, i.e., the patient, which bothwastes power and necessitates the use of a capacitor whose rated energystorage capacity is greater than the maximum energy deliveryrequirement, since energy is dumped into the dissipation resistors andnot recovered or otherwise put to therapeutic use.

Using a capacitor whose rated energy storage capacity is greater thanthe maximum energy delivery requirement is disadvantageous from severalperspectives. For a given capacitor technology, greater storage capacityrequires greater size and/or weight. A physically larger capacitor isundesirable, particularly for use in portable equipment, becauseequipment must be built larger and is more difficult to transport. Sizeand weight factors can prove prohibitive for equipment meant to be wornby, transported with, or carried by, a patient who themselves may not befully ambulatory. Moreover, energy that is wasted, yet must be stored onthe capacitor as described above, adds to the charging time and theperformance characteristics required of the charging circuit whichplaces the energy on the capacitor.

For example, in the conventional defibrillator described in theBACKGROUND section in which excess energy is dissipated into dissipationresistors, a capacitor having a minimum required energy rating ofapproximately 381 Joules is used. Under favorable conditions for maximumenergy shock (i.e., a 200 Joule setting into a patient presenting animpedance of 161Ω), approximately 69% of the capacitor's minimumrequired energy rating is delivered to the patient. For higher impedancepatients, energy utilization drops off slightly to 67% for a patientpresenting an impedance of 175Ω, and to 63% for a patient presenting animpedance of 200Ω. The drop off in energy utilization is more severe forlower impedance patients (e.g., 37% for a patient presenting a 25Ωimpedance, and 21% for a patient presenting a 15Ω impedance), primarilydue to energy dissipated in the dissipation resistors. One of thephysically smallest capacitors validated for use in such a conventionaldefibrillator weighs approximately 10 oz (283.5 grams) and has a volumeof approximately 20 in³ (327.7 cm³).

According to aspects of embodiments, a boost converter is employed tocontrol and regulate the delivery of a constant current, resulting in aconstant power dissipation level during the delivery of a desired energyimpulse. In brief summary, a boost converter transfers energy in veryshort bursts compared to the time for delivering the total desiredenergy impulse, first from the capacitor to an inductor, which storesthe energy as a substantially constant current, and then from theinductor to the patient. Because the current delivered to the patient bythe inductor is substantially constant due to the intrinsic electricalcharacteristics of inductors which tend to resist a change to currentthrough them, a constant, maximum desired power level is dissipated inthe patient, in accordance with the definitions of electrical powergiven above. A boost converter circuit of a defibrillator incorporatingaspects of embodiments is now described in greater detail.

First, the basic boost converter circuit is described in connection withFIG. 1. The boost converter circuit, 100, provides a substantiallyconstant current at its output node, 101, when that node is connected toa load, 102. The circuit, 100, includes a storage capacitor, 103, inwhich the energy for the desired impulse is held until a discharge intothe patient load is triggered; an inductor, 104, connected to receive acurrent from the storage capacitor, 103, when the discharge istriggered; a diode, 105, to protect against a reversal of the currentdischarge; and, optionally a smoothing capacitor, 106; as well ascontrol elements enumerated below. A charge circuit, such as a batteryor other DC power source (not shown) is coupled to the storage capacitor103, for example via relays, to provide energy to the storage capacitor103. A terminal of the storage capacitor 103 is electrically coupled toa first terminal of the inductor 104, which in FIG. 1 is modeled as aninductor 104 l coupled in series with a resistor 104 r. The secondterminal of the inductor 104 is electrically coupled to the anode of thediode 105, with the cathode of the diode 105 being electrically coupledto a first terminal of the optional smoothing capacitor 106 and to theoutput node 101.

According to the capacitor energy equation, E=½CV², an exemplarycapacitor, 103, of 270 g, as shown in FIG. 1, charged to about 1218 Vwould store 200 Joules for the exemplary therapeutic shock. To deliver200 Joules over a 10 msec impulse requires delivering a substantiallyconstant, instantaneous power of 20 kW to the load, 102. If the load,102, is 25Ω, then the power equation, P=I²R, calls for a current of 28A, while a load, 102, of 200Ω calls for a current of 10 A. Inductor,104, is of a size to prevent substantial current droop while deliveringa desired power level to the patient load, 102. A 1 mH inductor, 104, asshown in FIG. 1, produces the desired result, as illustrated below inFIGS. 2-6. In accordance with one embodiment, the inductor 104 may be anunsaturable 1 mH Litz wire air-core coil dimensioned to optimizeself-inductance.

While 100% utilization of the capacitor energy storage capability is thetheoretical goal, practical circuit elements, which have real lossesassociated with them, achieve somewhat lower utilization rates, per theTable I, below. The simulations presented in FIGS. 2-6, and discussedbelow, assume capacitor, 103, has a capacitance of 270 μF, and aninitial stored energy of 305 Joules.

TABLE I Patient Therapeutic 10 msec Initial Impedance Shock EnergyContinuous Power Energy Usage  25 Ω 249 Joules 24.9 kW 81%  50 Ω 255Joules 25.5 kW 83% 100 Ω 236 Joules 23.6 kW 77% 150 Ω 221 Joules 22.1 kW72% 200 Ω 210 Joules 21.0 kW 69%

By comparison to a conventional defibrillator using a storage capacitorhaving a minimum energy rating of 381 Joules, embodiments of the presentinvention permit the use of a storage capacitor having an approximately20% lower minimum energy rating (e.g., 305 Joules) while providing asimilar amount of energy to the patient. As a result, the size andweight of the storage capacitor 103 used with embodiments of the presentinvention may be reduced by approximately 20% relative to storagecapacitors used in a conventional defibrillator. Further efficiencies ofsize and cost are provided by eliminating the need for dissipationresistors and their associated shunting devices used in conventionaldefibrillators, as well as any of the thermal management features neededto dissipate the heat generated therefrom.

A specialized controller circuit, 107, modeled for convenience as aUC3842 current mode PWM controller, has a control output connected to acontrol input of a high-voltage and high-current, solid-state switch,108 that is coupled between the second terminal of the inductor 104 anda current sensing network 109. The solid state switch 108 may be an IGBTas shown in FIG. 1, or another type of a high-voltage and high currentsolid state switch, such as a thyristor. It should be appreciated thatembodiments of the present invention are not limited to the use of aparticular type of PWM controller or to a particular type ofhigh-current solid state switch, as other types of controller circuits,and other types of high-current switches may alternatively be used.Current drawn through the switch 108 is measured by the current sensingnetwork, 109; compared to a desired current profile, 110; and, theresult is provided as an input to the controller circuit, 107. Since, asexplained above, current is directly related by a square law toinstantaneous power, controlling for a desired current also controls forthe desired instantaneous power level.

The load presented by the patient, 102, is connected to the output node,101, through an H-bridge structure which causes current to flow throughthe patient in a desired direction at a desired time. The H-bridgeincludes four H-bridge switches 111, with each H-bridge switch 111 a,111 b, 111 c, 111 d including a respective switching transistor 116 a,116 b, 116 c, 116 d and a respective control circuit 117 a, 117 b, 117c, 117 d associated with each. The switching transistors can beinsulated-gate bipolar transistors (IGBTs), metal-oxide semiconductorfield-effect transistors (MOSFETs), silicon-controlled rectifiers (SCRs)or such other high-current switching devices as may be available. In theexemplary, illustrative embodiment, for modeling purposes only, anoscilloscope, 112, having a channel A input, 112A, and a channel Binput, 112B, has been included. Channel A, 112A, monitors the currentimpulses passed through the switch, 108, and channel B, 112B, monitorsthe voltage across the patient load, 102. The traces produced bychannels A and B, 112A and 112B, are shown in FIG. 2, which is nextreferred to in an explanation of the operation of the circuit of FIG. 1.

The circuit of FIG. 1 operates as follows to provide a 200-Jouledefibrillation shock to a patient load, 102, of 25Ω, as illustrated inFIG. 2. The storage capacitor, 103, is first charged up with about 200Joules of electrical energy, by a charging circuit (not shown). Thereare small, parasitic losses due to parasitic resistances throughout thecircuits which deliver the charge to the patient, including parasiticresistances in the inductor, 104 r, and elsewhere. If the parasiticresistances are negligible, then no more than about 200 Joules need bestored on the capacitor; however, if the parasitic resistances arenon-negligible, then the storage capacitor, 103, should hold a smallexcess above the desired 200 Joules of electrical energy, the excessbeing sufficient to just account for the energy dissipated in theparasitic resistances under worst-case conditions. It should beappreciated that other shock energies (i.e., other than 200 Joules) maybe provided, as known to those skilled in the art.

According to one embodiment, operation begins with the solid-stateswitch, 108, open, and each of the H-bridge switches, 111 a-d, open.When a therapeutic shock is triggered, a pair of the H-bridge switches,e.g., 111 a and 111 c, is closed, initiating current through the patientload 102. Current then builds up in the inductor, 104. As shown in FIG.2, indicated by line, 200, current through switch, 108, is zero duringthis initial period, 201. Next, during period, 202, the controllercircuit, 107, begins cycling switch, 108, on and off, thereby allowingcurrent through switch, 108. When the switch, 108, is closed and thecontroller circuit, 107, detects that the desired current or higher isflowing through the switch, 108, it provides a control signal to thesolid-state switch, 108, to again open the switch, 108, allowing currentthrough the inductor, 104, to the patient load, 102. At regularintervals, the controller, 107, closes the switch, 108, and checks forthe current to build up to the desired level, at which point the controlsignal again opens the switch, 108.

During each cycle, during period, 202, when the controller circuit, 107,determines from the output of the current sensing network, 109, that thecorrect current level has been reached or exceeded, a control signal isapplied to the solid-state switch, 108, to open the switch, allowingcurrent through the inductor, 104, and the patient load, 102, from theenergy stored on storage capacitor, 103. As current is initiated throughthe patient, a voltage, indicated in FIG. 2 by line 210, appears acrossthe patient that causes the current in the inductor, 104, to begin todecay, and so the controller circuit, 107, again closes solid-stateswitch, 108, to begin the cycle again by building up the current storedin the inductor. By repeating the forgoing cycle many times during thetherapeutic shock, energy stored on the storage capacitor, 103, ismetered out to the patient without ever exceeding the maximum allowablepower dissipation level in the patient. According to some embodiments,it has been found that the desired waveform to be applied to the patientreverses polarity after an interval. Accordingly, the H-bridge switchesare controlled by a desired phase profile, 120, to open the closed pairof switches, 111 a and 111 c, and close the open pair of switches, 111 band 111 d, at about 6 msec into the therapeutic shock cycle, reversingthe polarity of the applied shock, 211. It should be noted that switches111 a and 111 c are opened prior to closing switches 111 b and 111 d toavoid short-circuiting the H-bridge structure. The magnitude of thecurrent applied to the patient load, 102, (and the resulting voltageacross the patient load, 102) does not substantially change during thepolarity reversal, 211 and 213.

As shown in FIGS. 3, 4, 5, and 6, the operation is similar for patientspresenting resistance values of 50Ω, 100Ω, 150Ω, and 200Ω. The variationin the load, 102, results in different damping characteristics for thetherapeutic shock waveforms, i.e., the overall shape of the waveform,and also results in different patient voltages, such that the 200 Jouleimpulse is applied as desired. In each of FIGS. 3, 4, 5, and 6,reference numerals indicating corresponding elements to elements of FIG.2 correspond, except for the hundreds place, which corresponds to theFIG. number. For example, FIG. 2, line 200, corresponds to FIG. 3, line300, but for a different patient load, 102.

As shown in FIG. 2, for a patient load, 102, of 25Ω, the absolute valueof patient voltage 210 varies between a peak of about 1.5 kV and 500 V.For a patient load, 102, of 50Ω, the absolute value of patient voltage310 has a much flatter shape, as shown in FIG. 3. It hits a peak ofabout 2 kV but remains for most of the impulse at about 1.2 kV, finallytapering down to just under 1.0 kV. As shown in FIG. 4, for a patientload, 102, of 100Ω, the absolute value of patient voltage 410 has aneven flatter shape. It also hits a peak of about 2 kV but remains formost of the impulse at about 1.5 kV, finally tapering down to about 1.2kV. For a patient load, 102, of 150Ω, the absolute value of patientvoltage 510 has a quite flat shape, as shown in FIG. 5. It hits a peakof about 2 kV but remains for nearly the entire impulse at about 1.8 kV.For a patient load, 102, of 200Ω, the absolute value of patient voltage610 has a quite flat shape. It hits a peak of about 2 kV but remains fornearly the entire impulse at about 1.8 kV, as shown in FIG. 6.

In practical systems, the preference is to deliver substantiallyconstant energy to the patient during a period of time. Thus, if a 200Joule therapeutic shock is desired to be delivered in a 10 msec period,the controller circuit, 107, is designed or programmed to obtain acurrent level in the inductor, 104, that delivers 20 J/msec. Thecontroller circuit, 107, in connection with the current sensing network,109, the desired current profile, 110, and the solid state switch, 108,forms a feedback loop that controls and maintains the 20 J/msec level,or such other level or waveform as desired. Except for parasitic losses,explained below, the storage capacitor, 103, need only have a ratedenergy storage capacity of 200 Joules, since no excess energy is dumpedand it is desired to leave no residual energy in the storage capacitor,103, after the therapeutic shock has completed.

FIGS. 7A-F illustrate the manner in which the boost converter circuit100 of FIG. 1 may be used to control the shape and/or phase of thedefibrillating waveform applied to the patient. As described previouslywith respect to FIG. 1, current drawn through the switch 108 is measuredby the current sensing network 109, compared to a desired currentprofile 110, and provided as an input to the controller circuit 107. Inthe circuit illustrated in FIG. 1, this comparison is performed by anoperational amplifier 118 configured as a comparator. By controlling theshape and amplitude of the desired current profile 110, a waveform of adesired amplitude and desired shape may be delivered to the patient load102.

For example, as shown in FIG. 7A, a current profile 110 having a stepimpulse that corresponds to a current level of approximately 22 A andthat gradually increases to a level corresponding to approximately 40 Amay be used to provide the current and voltage waveforms depicted inFIGS. 3-6. The current profile 110 is approximately 10 msec in durationand includes an initial period or region 110 a where the current is zero(corresponding to periods 301, 401, 501, and 601 in FIGS. 3-6), followedby a step impulse (region 110 b) corresponding to approximately 22 A. Atapproximately 1 msec, the current profile 110 linearly increases (region110 c) to a value corresponding to approximately 24 A. The currentprofile 110 remains at a level corresponding to about 24 A for about 2msec (region 110 d), where it then linearly increases (region 110 e) toa level corresponding to approximately 40 A, after which the currentprofile returns to a zero current level (region 110 f) at approximately10 msec. The overall shape of the current profile 110 used to generatethe current and voltage waveforms depicted in FIGS. 3-6 is shown in eachrespective figure by the respective envelope or profile 303, 403, 503,and 603 of the current through the switch 108. Differences in thevoltage waveforms 310, 410, 510, and 610 applied to the patient in FIGS.3-6 are primarily due to differences in the patient load 102. Theoverall shape of the current profile 110 used to generate the currentand voltage waveforms depicted in FIG. 2 is shown in FIG. 2 by theenvelope 203 of the current 200 through the switch 108.

The phase of the defibrillating waveform that is applied to the patientmay be controlled by the desired phase profile 120 provided to each ofthe H-bridge switches 111. For example, FIG. 7B illustrates a desiredphase profile 120 that may be used to control the phase of the waveformsdepicted in FIGS. 2-6. As shown, the phase profile 120 initially assumesa high state at time zero (or before) followed by a change to a lowstate at 6 msec. In the circuit depicted in FIG. 1, each of H-bridgeswitches 111 a and 111 c receives the phase profile 120, while each ofH-bridge switches 111 b and 111 d receives an inverted version of thephase profile 120. The high state of the phase profile 120 operates tofully close each of H-bridge switches 111 a and 111 c during the initial6 msec, and to maintain each of switches 111 b and 111 d in a fully openposition. At approximately 6 msec, the level of the phase profile 120changes, thereby fully opening H-bridge switches 111 a and 111 c, andfully closing switches 111 b and 111 d, thereby reversing the polarityof the delivered voltage waveform as shown in each of FIGS. 2-6. Thepresence of the inverter 115 serves to not only invert the phase profile120, but to delay the signal provided to each of H-bridge switches 111 band 111 d to help ensure that these switches are not closed until afterswitches 111 a and 111 c have opened. If necessary, additional delayscould be provided. Where the switching transistors 116 a, 116 b, 116 c,and 116 d used in each of the H-bridge switches 111 a, 111 b, 111 c, 111d are capable of operating in a linear mode, the high state and the lowstate of the phase profile 120 should be such that the switchingtransistors are either fully conducting (on) or fully non-conducting(off) to avoid thermal destruction.

FIG. 7C illustrates an alternative desired current profile 110 that maybe used with the boost converter circuit 100 of FIG. 1 to generate abiphasic voltage waveform that increases asymptotically from a zerovalue to a desired voltage level (e.g., to approximately 2 kV inamplitude for a 200Ω patient) over the initial portion of each phase.When combined with a phase profile 110 similar to that illustrated inFIG. 7B, the boost converter circuit 100 may provide a defibrillatingvoltage 710 to the body of the patient similar to that shown in FIG. 7D.As shown in FIG. 7D, the biphasic voltage waveform 710 is approximately10 msec in duration and switches phase at approximately 6 msec. Duringthe first few milliseconds of each phase, the voltage 710 applied to thebody of the patient rises asymptotically to an amplitude of about 2000 V(for a 200Ω patient). To achieve the shape of the voltage waveform 710shown in FIG. 7D, the smoothing capacitor 106 (FIG. 1) may be omitted orset to a value of zero. Such a ramped asymptotic voltage waveform asshown in FIG. 7D may reduce the amount of trauma to the patient's heartduring defibrillation, by avoiding the step impulse in voltage shown ineach of FIG. 2-6.

FIGS. 7E and 7F illustrate alternative phase profiles that may be usedwith the current profile 110 discussed above with respect to FIG. 7A.For example, FIG. 7E illustrates a desired phase profile 120 having twoopposing phases over a 10 msec duration, in which the opposing phasesare substantially similar in duration (i.e., about 5 msec each). Such aphase profile may be used to balance the amount of charge delivered tothe patient's heart in each direction and thereby potentially reduce thetrauma to the patient's heart. FIG. 7F illustrates yet an alternativephase profile that may be used to provide a monophasic defibrillatingshock to the body of the patient.

It should be appreciated that a variety of different current profiles110 and phase profiles 120 may be used with the boost converter circuit100 of FIG. 1 to generate a corresponding variety of differentdefibrillating waveforms, each having a different shape and/or amplitudeand/or phase. For example, the current profile 110 could include asimple exponential waveform or a damped sine wave, and the phase profile120 could be monophasic, biphasic, triphasic, or otherwise. Accordingly,where it is determined that a particular shape, amplitude, phase,sequence of phases, or all of the above is particularly effective,reference waveforms may be generated and used as desired current profile110 and desired phase profile 120 to achieve the desired resultantdefibrillating waveform. Although not depicted in FIG. 1, each of thedesired current profile 110 and the desired phase profile 120 may bestored in a memory, and provided, for example, by a processor of thedefibrillator to comparator 118 and each of H-bridge switches 111 tovary the shape, amplitude, or phase of the defibrillating waveformapplied to the patient as desired.

Referring back to FIG. 1, again, within each H-bridge switch, 111 a-d,is a driver circuit (i.e., 117 a, 117 b, 117 c, and 117 d). Thesecircuits are feedback systems illustrated in greater detail in FIG. 8.Each driver circuit is configured to control a switching transistor(i.e., 116 a, 116 b, 116 c, and 116 d, respectively) to pass a currentup to a controlled maximum level above which the current is clipped.

In each driver circuit is an operational amplifier, 802, connected toreceive a control signal, DRIVE, that is based upon the desired phaseprofile 120 and produce an output, GATE, which turns a switchingtransistor (FIG. 1, 116 a, 116 b, 116 c, and 116 d) on and off asrequired. A second operational amplifier, 804, is connected to sensecurrent through each switching transistor (FIG. 1, 116 a, 116 b, 116 c,and 116 d), and control an input to operational amplifier, 802, so avoltage at output, GATE, turns the switching transistor on up to adesired maximum current through the switching transistor. In theembodiment depicted in FIG. 1, each of the switching transistors 116 a,116 b, 116 c, and 116 d is an IGBT that may be operated in a non-linearmode as a two state (i.e., on or off) switch, or in a linear mode as avoltage controlled current source. Where the circuit 100 is used toprovide therapeutically effective amounts of energy sufficient fordefibrillation, the switching transistors 116 a, 116 b, 116 c, and 116 dwould typically be operated in the non-linear mode (e.g., as a two stateswitch) to avoid thermal destruction. However, by adjusting the timingdefined by controller circuit, 107, the desired current profile, 110,and the desired phase profile, 120, the circuit of FIG. 1 can beprogrammed to perform pacing, as well as defibrillation. For example, byappropriately controlling the timing defined by the controller circuit107 and the desired current profile 110, and at energies typically usedfor pacing, the smoothing capacitor 106 can substantially correspond toa DC power supply of a desired voltage. By then providing a suitablephase profile to pairs of H-bridge switches (i.e., 111 a and 111 c, 111b and 111 d), the switching transistors of a respective pair of H-bridgeswitches may be operated in their linear mode to provide a desiredpacing waveform.

FIGS. 9 and 10 illustrate a phase profile 120 that may be provided topairs of H-bridge switches, 111 a and 111 c or 111 b and 111 d, todeliver suitable pacing waveforms 902, 1002, to a patient load 102, whenthe timing defined by the controller circuit 107 and the current profile110 have been appropriately programmed In each of FIGS. 9 and 10, thetiming of the controller circuit 107 and the current profile 110 havebeen programmed so that smoothing capacitor 106 effectively operates asa 50 V D.C. voltage source. As illustrated in FIG. 9, a voltage level ofapproximately 27 mV provided to the DRIVE input of driver circuits 117Aand 117C is sufficient to deliver an 8 mA, 200 mV pacing pulse into apatient 102 presenting a 25Ω load. As illustrated in FIG. 10, a voltagelevel of approximately 465 mV is sufficient to deliver an 140 mA, 42 Vpacing pulse into a patient 102 presenting a 300Ω load. By appropriatelycontrolling the phase profile 120, pacing pulses ranging from a fewmilliamps to two hundred milliamps or more may be provided to a patient,using the same circuit topology as that used to deliver a defibrillatingshock.

It should be appreciated that although embodiments of the presentinvention have been primarily described with respect to defibrillationand pacing, they may also be used to deliver other types of therapeuticwaveforms to the body of a patient in which the energy delivered isbetween the ranges of energy typically used for pacing ordefibrillation. For example, pacing pulses typically range from a few mAto approximately 200 mA, and defibrillation pulses typically range fromabout 1 A to about 35-40 A. Between these ranges of current exists awide spectrum of energies that may be applied to the body of a patientfor a variety of therapeutic purposes, for example, to perform chargebumping of a patient's heart, etc. Accordingly, by varying the timing ofthe controller circuit, the current profile 110, and the phase profile120, embodiments of the present invention may be used to tailor one ormore of the shape, the voltage, and the current of a therapeuticwaveform to be applied to the body of a patient.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the scope of theinvention. Accordingly, the foregoing description and drawings are byway of example only.

What is claimed is:
 1. A system to deliver therapeutic energy to apatient, the system comprising: a capacitor configured to store andrelease the therapeutic energy; a converter circuit coupled to thecapacitor and constructed to control a shape of a therapeutic waveformthat arises upon release of the therapeutic energy from the capacitor,the converter circuit including: an inductor, a current sensing network,coupled to the inductor, to sense a magnitude of current in theinductor, a switch coupled between the inductor and the current sensingnetwork, and an operational amplifier configured to compare themagnitude of current in the inductor with a current profile, wherein theconverter circuit is constructed to control a state of the switch basedon the comparison; and a current flow control circuit coupled to theconverter circuit and comprising a plurality of control circuitsconfigured to linearly control a current output from the current flowcontrol circuit upon release of the therapeutic energy from thecapacitor.
 2. The system of claim 1, wherein the current flow controlcircuit comprises a plurality of transistors arranged as an H-bridgecircuit.
 3. The system of claim 2 wherein each control circuit of theplurality of control circuits is configured to control a gate voltage ofswitching respective transistor of the plurality of transistors tolinearly control the current output from the current flow controlcircuit.
 4. The system of claim 2 wherein the plurality of transistorscomprises one of insulated-gate bipolar transistors, metal-oxidesemiconductor field-effect transistors, and silicon-controlledrectifiers.
 5. The system of claim 2 wherein the plurality of controlcircuits is configured to control each transistor of the plurality oftransistors such that each transistor provides a voltage controlledcurrent source.
 6. The system of claim 1 wherein the plurality ofcontrol circuits is configured to linearly control the current outputfrom the current flow control circuit such that the current flow controlcircuit is configured to provide both of a defibrillation waveform and apacing waveform.
 7. The system of claim 1 wherein the therapeuticwaveform is a ramped biphasic waveform that increases to a desiredvoltage level over an initial portion of each phase.
 8. The system ofclaim 7 wherein the initial portion of each phase has a duration of oneto six milliseconds and a voltage of the therapeutic waveform graduallyincreases from a zero voltage to a target voltage over the duration ofthe initial portion of each phase.
 9. The system of claim 1 wherein thecurrent flow control circuit is configured to control the therapeuticenergy delivered to the patient without providing a current path throughdissipation resistors.
 10. The system of claim 1 wherein the capacitoris a single capacitor having a minimum energy rating of approximately305 Joules.
 11. The system of claim 1 wherein, for a patient impedanceof 200 ohms, a utilization of a capacitor energy storage capability isat least 69%.
 12. The system of claim 11 wherein, for the patientimpedance of 200 ohms, the utilization of the capacitor energy storagecapability is less than 100%.
 13. The system of claim 1 wherein, for apatient impedance of 150 ohms, a utilization of a capacitor energystorage capability is at least 72%.
 14. The system of claim 13 wherein,for the patient impedance of 150 ohms, the utilization of the capacitorenergy storage capability is less than 100%.
 15. The system of claim 1wherein a utilization of a capacitor energy storage capability isapproximately 65%-75% for a patient impedance of approximately 150-200ohms.
 16. The system of claim 1 wherein the shape of the therapeuticwaveform controlled by the converter circuit is represented by a patientvoltage as a function of therapeutic energy delivery time and wherein anabsolute value of the patient voltage exhibits a flatter shape for ahigher impedance patient load than for a lower impedance patient load.17. The system of claim 1 wherein the current output from the currentflow control circuit cyclically varies as a function of therapeuticenergy delivery time and wherein a shape of an envelope profile of thecyclically varying current flow is a function of patient impedance anddetermines the shape of the therapeutic waveform.